1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine.
2. Description of the Related Art
A called creeping discharge type spark plug has been known as the spark plug for an internal combustion engine, which is improved in anti-contamination property. The spark plug is designed such that a spark generated in the spark gap propagates, in a creeping discharge fashion, on and along the surface of the insulating member constantly or when a specific condition is satisfied. A called semi-creeping discharge type spark plug includes a center electrode, an insulating member disposed surrounding the center electrode, and a ground electrode having the top end of which the firing surface is opposed to the side surface of the center electrode. The top end of the insulating member is located in a space (i.e., a spark discharge gap) between the center electrode and the firing surface of the ground electrode. In a creeping discharge, spark creeps on and along the surface of the top end of the insulating member, while a gaseous discharge is performed between the firing surface of the ground electrode and the surface of the insulating member. When the spark plug is used for a long time in a state that the electrode is at low temperature of 450xc2x0 C. or lower, as in a pre-delivery, a called xe2x80x9ccarbon foulingxe2x80x9d or xe2x80x9cwet foulingxe2x80x9d state is set up, and the insulating member is covered with conductive contamination materials, such as carbon. As a result, the spark plug tends to improperly operate. Meanwhile, in the creeping discharge type spark plug, spark is generated creeping on the surface of the insulating member. Therefore, the contamination materials are constantly burnt out. In this respect, this type of the spark plug is improved over the gaseous discharge type spark plug in the anti-contamination property.
In the creeping discharge type spark plug, as known, spark creeping on the surface of the insulating member is frequently generated, and a called channeling phenomenon in which the surface of the insulating member is grooved is easy to occur. When the channeling progresses, the following disadvantage is likely to occur: deterioration of the heat resistance and reliability of the spark plug. The channel is easy to occur when the engine is operated at high speed or high load. With recent engine power increase, the market needs spark plugs with good durability. Accordingly, the requirements for the channeling prevention or restriction are stricter.
Accordingly, an object of the present invention is to provide a spark plug which is excellent in anti-contamination property, hard in channeling, and good in durability.
In order to solve the above-described problems, a spark plug according to a first aspect of the present invention comprises:
a center electrode;
an insulating member-disposed around said center electrode in a state that a top end of said center electrode is exposed at a top end of said insulating member;
a main metallic shell provided covering said insulating member;
a ground electrode forming spark discharge gap between the top end of said center electrode and a top end of said ground electrode, and being positioned relative to the top end of said insulating member and the top end of said center electrode so as to allow creeping discharge to be performed along the surface of the top end of said insulating member; and
a corrosion impeding layer formed on the surface of the top end of said insulating member, whereby corrosion of the surface of the top face of the insulating member, caused by the creeping discharge, is restricted.
The corrosion impeding layer thus formed protects the insulating member from the attack by the creeping discharge spark. And it significantly effectively prevents or restricts the channeling.
The corrosion impeding layer can be previously formed on the surface of the top end portion of the insulating member prior to using the spark plug. Alternatively, the center electrode and/or the ground electrode is constituted to contain the forming component of the corrosion impeding layer, so that the corrosion impeding layer containing the forming components of the electrodes is naturally formed on the surface of the top end portion of the insulating member with progression of spark discharge in the spark discharge gaps. Needless to say, it is possible to use both methods together. Particularly, in the latter method, even if the corrosion impeding layer is gradually wasted by spark attack, a new corrosion impeding layer can be formed accompanying with continuous using of the spark plug, while the electrodes is used as the component supplying source. Accordingly, the latter method is excellent for maintaining the effect.
The corrosion impeding layer can be constituted to contain at least one of Fe, Cr and Cu as an insulating member corrosion impeding component. Accordingly, it is possible to further enhance the effects for protecting the insulating member from attack of the creeping discharge spark and for preventing or restricting channeling.